Photosensitive materials generally consist of an electrically insulating base and a photographic layer. Accordingly, static charges often accumulate during production of photosensitive materials or during use. The charges are created by friction between surfaces of similar or different kinds of materials or separation thereof. The accumulated static charges create various bad effects. The most serious effect is that static charges which accumulate before the development processing are discharged exposing the sensitive emulsion layer. This allows for the production of dot spots or branched or fur-like specks on the photographic films when subjected to development processing. They are the so-called "static mark", which may markedly damage or completely destroy the commercial value of the photographic films. Extremely serious effects may occur if static marks appear on medical or industrial X-ray films. This phenomenon is particularly troublesome, because it does not become evident until development is carried out. Further, the accumulated static charges can cause secondary troubles such as dust adheres to the surface of the film or preventing the formation of a uniform coating.
As mentioned above, static charges are often accumulated during production or use of photosensitive materials. For example, during production static charges may be generated by friction between the photographic film and rollers or by detachment of the base face from the emulsion face during winding or rewinding of the photographic film. With respect to finished products, static charges may be generated by detachment of the base face from the emulsion face when the photographic film is rewound or by contact of the X-ray film with machine parts in an automatic camera or with fluorescent sensitizing paper or by separation from them. In addition, they may be generated by contact with wrapping materials. The possibility of having a static mark on photosensitive materials due to the accumulation of static charges increases greatly with an increase in the sensitivity of the photosensitive materials and an increase of the processing rate thereof. Recently, photosensitive materials are even more likely to be highly sensitized and be subjected to severe treatment such as high-speed coating, high-speed photographing or high-speed automatic processing, etc. Consequently, the static marks are more likely to be generated.
The preferred process for removing static electricity is to improve the electric conductivity of the materials so that the static charges disappear for a short period of time before the accumulated charges are discharged. Accordingly, proposed methods involve improving the electric conductivity of the base or various coating surface layer of the photosensitive materials using various hygroscopic substances, water-soluble inorganic salts, certan kinds of surface active agents and polymers. For example, it has been known to use polymers described in U.S. Pat. Nos. 2,882,157, 2,972,535, 3,062,785, 3,262,807, 3,514,291, 3,615,531, 3,753,716 and 3,938,999; surface active agents described in U.S. Pat. Nos. 2,982,651, 3,428,456, 3,457,076, 3,454,625, 3,552,972 and 3,655,387; and colloidal silica described in U.S. Pat. No. 3,525,621.
However, many of these materials show singularity depending on the kind of film base and differences in photographic composition. They may produce good results on certain kinds of film base and photographic emulsion or on other photographic composition elements. However, while they are not only entirely useless on other film bases or other photographic composition elements, they have a bad effects upon photographic properties. Furthermore, many of these materials cannot operate effectively as an electrically conductive layer under low humidity.
A meothd of using stannic oxide as an antistatic agent is described in Japanese Patent Publication No. 6616/60. In this method, a colloid of amorphous stannic oxide is used. However, the electric conductivity of amorphous stannic oxide is humidity dependent and cannot operate effectively under low humidity. Accordingly, it is not essentially different from the various above-described materials.
U.S. Pat. No. 3,062,700 and Japanese Patent Application (OPI) Nos. 113224/77 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") and 12927/80 disclose crystalline metal oxides such as zinc oxide, stannic oxide or indium oxide which possess electric conductivity not dependent on humidity. These metal oxides are used as an electrically conductive material for a conductive base of electrophotographic sensitive materials or electrostatic recording materials. However, these crystalline metal oxides have not been disclosed as being useful as antistatic agents for silver halide emulsions. Furthermore, it is not possible to determine what sort of interaction might occur between these oxides and a silver halide photosensitive emulsion layer. In support of this statement, it should be pointed out that silver halide and copper halide are disclosed as being used as the electrically conductive materials in U.S. Pat. No. 3,245,833. However, these electrically conductive materials interact with the silver halide emulsion layer as described in U.S. Pat. No. 3,428,451 resulting in bad effects upon photographic properties.
Images are obtained with the use of photosensitive material via transmitted light or reflected light. In the latter case, the reflectivity of the non-image part is preferably as high as possible. On the other hand, in the former case, it is preferable to increase light transmittance of the non-image part, and reduce light scattering caused by the photosensitive material. When attempting to prevent the generation of static charges by introducing an electrically conductive layer into the photosensitive material, there are significant restrictions with respect to reducing the light scattering. Consequently, under existing circumstances, the above-described electrically conductive polymers or surface active agents (which are humidity dependent with respect to electric conductivity) can only be utilized by dissolving them in a binder or by forming a fine micelle state. Accordingly, even though the above-described zinc oxide, electrically conductive stannic oxide and indium oxide described in U.S. Pat. No. 3,062,700 and Japanese Patent Application (OPI) Nos. 113224/77 and 12927/80 can be utilized for electrically conductive paper for electrophotography or electrostatic recording, they cannot be used directly for silver halide photosensitive materials in which the images formed are observed by transmitted light.